In Nigeria, the military has been battling Islamist militants in the nation's north. Troops appeared to have driven the militants out of the northeastern city of Maiduguri - that is, until Monday. Hundreds of insurgents launched a deadly attack on an air force base on the outskirts of the city. They also struck the airport.

The extremist group known as Boko Haram is being blamed for the raid. And there's talk of possible collusion from military and paramilitary personnel. The U.S. branded Boko Haram a terrorist organization last month.

NPR's Ofeibea Quist-Arcton is in Nigeria's main city, Lagos, and she joins me now. And, Ofeibea, tell us more about this attack that came on Monday.

OFEIBEA QUIST-ARCTON, BYLINE: First of all, let me say, Melissa, that there hasn't been an attack on Maiduguri for many months. So, residents there were beginning to feel a little, if not comfortable, at least that their security was more assured since the military crackdown in May, when a state of emergency was imposed by the government against the insurgency.

So, to wake up on Monday to the sound, we're told, of artillery shells and a racket, as these suspected militants attacked the air force base and a barracks - and briefly, the airport in Maiduguri - is a huge shock to residents. And then to find the destruction also in the city itself, we were told the main roads strewn with shells, charred buildings, charred vehicles, aircraft.

People of Maiduguri are in deep shock. They say how come? The military have told us that they are driving the militants into the rural areas. And yet, look what happened.

BLOCK: It does seem especially brazen, Ofeibea, that this attack came not just in the city, but also on a military target. They attacked an air force base.

QUIST-ARCTON: In Maiduguri, which is the provincial capital of the northeast of Nigeria. And, as you said in the introduction, Melissa, the military is saying this smells of an insider job. Who is helping these suspected Boko Haram fighters - Boko Haram, meaning Western education - is Haram for militants? This uprising has said that it wants to see Islamic Sharia law imposed in Nigeria - strict Sharia law. And the government saying this is not the way.

Now, President Good Luck Jonathan says the way forward is to boost the economy of the northeast because that is the way to end youth unemployment, poverty - the root causes that people have been saying for years are the problems that have led to this insurgency. These are the issues Nigeria has to grapple with.

BLOCK: What's the Nigerian government's response been overall to the extremist groups? And what can they do?

QUIST-ARCTON: Well, talking more widely, just today, an investigative team - which has been set up by Nigeria's Defense Quarters - has issued a report recommending the immediate trial of 500 detainees, not necessarily linked to Monday's attack. The Defense spokesman said these include, in quotes, "high-profile suspects" who trained terrorists and weapons handling, as well as insurgents trained across not so far in Mali and other countries, for the purpose of perpetuating terror in Nigeria.

And he says among the suspects, a medical doctor, paramilitary or military service personnel who are fighting on the side of the terrorists. So this is an issue that has not been resolved. But the U.S. saying that detainees must be charged and brought to court; and it's interesting that this comes out the week the U.S. Assistant Secretary Linda Thomas Greenfield is here in Nigeria.